A parabolic reflector is a constituent element of a microwave frequency off-set parabolic antenna. Taking advantage of the unique property of that dish shaped reflective surface, wherein RF energy incident at any location on the surface is reflected to the parabola's focal point, the antenna's feed component is located at the surface's focal point. As a consequence, the more diffuse essentially spatially displaced RF fields propagating through space are concentrated or focused to a single point, thereby producing a more intense RF field at that point. That advantage permits intelligible reception of weaker RF signals than otherwise could not be detected. For the foregoing reason and other reasons well known to those skilled in the art, the parabolic antenna is widely used in communications systems, including those found in space vehicles.
In space vehicle application those antennas are "deployable". That is, the antenna is constructed of a structure that may be folded up into a package of small volume, suitable for stowage in the limited space available on board a space craft and then be expanded to a much larger size structure, following launch and orbital positioning of that space craft. RF deployable parabolic reflectors in space vehicle application typically employ a reflective fabric, such as a elastic wire mesh or comparable structure, as the reflective surface. The reflective fabric is light weight and pliant in nature, so it may be compacted as part of the stowed package. When the reflector is deployed, the fabric is stretched out by the associated supports to form a parabolic curved surface. An example of such reflectors are found in the patent literature, such as U.S. Pat. No. 5,680,145 granted Oct. 21, 1997 to Thomson et al, assigned to Astro Aerospace Corp.
A more recent deployable antenna is known as a foldable perimeter truss antenna, such as the one described in the '145 patent to Astro. In that type antenna the reflective material is supported upon a truss, a framework of tubes that is formed into a short hollow cylinder. The reflective material covers a circular end of that hollow cylinder and lies there over, vaguely resembling a sagging drum head.
To support and profile the shape of the pliant reflective mesh material, lines, referred to as catenaries, are strung from the periphery of the cylinder across the end and collectively define a skeletal parabolic shape. The reflective material is then tied to those catenaries and assumes the parabolic shape defined by that skeletal structure. The foregoing only generally describes the truss structure and its connection to the supported reflective material. Those less skilled in the art who wish to ascertain additional details of that structure are invited to review the cited patent to Thomson et al. Additional reference may also be made to the novel perimeter truss structure described in the pending application to Gilger and Parker, the present inventors, Ser. No. 09/080,767 filed May 18, 1998 and now U.S. Pat. No. 6,028,570, granted Feb. 22, 2000.
The present improvement relates to the catenaries and the connection of those catenaries to the truss structure and to the resultant effect thereof on the resultant profile of the reflective mesh supported on those catenaries. More particularly, the foregoing reflector structure includes an adjustment scheme to permit factory adjustment of the contour of the mesh.
Essentially a large number of cords or ties attached to the backside of the reflective fabric are used to attach the reflective mesh fabric to the supporting catenaries. The mesh is essentially sewn to each catenary line.
In practice, to form the desired contour or profile in one prior truss structure, two identical sets of catenary lines are used. One set is attached to the front end and the other set on the rear end, with each catenary line in one set being aligned with and overlying a corresponding catenary line in the other set. Each set is formed of a plurality of spaced lines that extend across the circular end of the truss. The lines ends are attached to a structural member along the periphery of the truss, so that each line is supported like a clothes line, and held reasonably taut.
At each intermediate positions along each line, a cord or tie of a precisely determined length for that position is dropped down and tied to the same intermediate position on the underlying catenary line of the second set. The tie pulls one catenary line against the other identical line, pulling the line at the front end down and the line at the rear end up similar to the vertical lines on a suspension bridge. By appropriately adjusting the length of each tie in the series on that one catenary line, the associated catenary line in the two sets are formed to a parabolic shape, forming essentially a section line of an imaginary parabolic surface. By making the same ties and adjustment in the ties of all of the other catenary lines to form section lines at different positions of such an imaginary parabolic surface, the net effect is that the front catenary lines, and the rear ones as well, collectively define a skeletal configuration of the desired parabolic shape.
The reflective metal mesh material is of a generally circular shape at least as large as the circular front end of the truss. It is fastened along its outer edge to a structural support along the periphery of the truss, leaving the central portion of the material to drape. Being somewhat pliant, the central portion of the metal mesh drapes into place onto the catenaries, which serves as its bed, and assumes the parabolic contour or profile defined by those catenaries. The mesh is then sewn to the catenary lines to form a permanent attachment.
The foregoing describes but one arrangement for the catenary lines and the mesh and serves to introduce the general principles underlying the catenary arrangement and the relationship to the latter of the reflective mesh. In another arrangement the reflective material is located beneath the upper catenaries, held taut about its periphery by supports along the periphery of the support, and the catenary drop ties are threaded through the reflective mesh and connected to the corresponding catenary lines on the rear set. In achieving its parabolic contour, the upper catenary lines press against the reflective mesh, and press the mesh into a like contour. There are of course others, such as that presented in the cited Astro patent, and new structures such as presented in the cited pending application of Gilger et al, but the general principles of the catenaries are common to all.
In the described way, the contour of the surface is adjusted to the desired geometric shape. The foregoing is a hand adjustment. The more precision in shape an antenna requires, the greater the number of catenaries and/or drop ties used, and, thus, the greater then is the number of adjustments needed to attain the desired accuracy. Typically, the number of ties that must be adjusted numbers in the hundreds.
As one appreciates, assembly and adjustment of the ties is tedious, time consuming and difficult, unlike the ease with which review of the foregoing text was made. The fabric and ties are physically delicate in nature. Contributing to the difficulty of the manual adjustment is the risk that the technician may inadvertently damage the reflector. Hence, the technician must be extra cautious in performing the large number of adjustment operations required. That caution is translated into slowness of production. And should damage occur, the assembly must be repaired, which is time consuming, or be entirely abandoned and construction started over. Moreover, the completion of a perfectly shaped reflector at the factory is not an end to the problem. The reflector must be delivered and deployed.
Moving from a successful adjustment at the factory to deployment in outer space introduces many additional parameters that may adversely affect the factory adjustments, a change in gravitational force, as example, and thereby change the accuracy of the reflective surface's profile. No existing means is known to adjust the surface's contour once the reflector has been deployed in outer space. And with a less than ideal profile for the reflective surface, the parabolic antenna's RF gain is reduced from the optimum level, lowering performance.
Accordingly, an object of the invention is to provide a hands-free adjustment scheme for adjusting the contour or profile of a pliable reflective surface.
A further object of the present invention is to enhance the efficiency with which a deployable reflector may be manufactured.
A still further object is to improve the contour adjustment procedures for a foldable perimeter truss reflector antenna.
An additional object of the invention is to permit remote control of the profile adjustment of a deployable offset or symmetrical parabolic antenna's reflector, allowing the contour to be adjusted or refocused even after deployment in outer space.